Electroacoustic transducer



March 7, 1967 F. MASSA, JR 3,308,423

ELECTROACOUSTI C TRANSDUCER Filed Dec. 30, 1963 3 Sheets-Sheet lINVENTOR.

FRANK MA SSA JR. BY 2 March 7, 1967 F. MASSA, JR 3,308,423

ELECTROACOUSTIC TRANSDUCER Filed Dec. 30, 1963 3 Sheets-Sheet 2 m My H![L IN VENTOR. FRANK MA SSA, JR

March 7, 1967 F. MASSA, JR

ELECTROACOUSTIC TRANSDUCER Filed Dec. 30, 1963 3 Sheets-Sheet 5INVENTOR. FRANK M45514, JR.

United States Patent Office 3,368,423 Patented Mar. 7, 1967 3,308,423ELECTROACOUSTHC TRANSDUCER Frank Massa, .lr., Cohasset, Mass assignor,by mesne assignments, to Dynamics Corporation of America, New York,N.Y., a corporation of New York Filed Dec. 30, 1963, Ser. No. 334,203 8Claims. (Cl. 340-8) This invention relates generally to improvements inelectroacoustic transducers, and, more particularly, to a new andimproved type of electroacoustic transducer structure for permittingreliable under-water operation in the low and midaudible frequencyrange.

Those skilled in the art of under-water transducer design willappreciate that severe problems must be overcome to obtain efficientacoustic radiation under water at the lower frequencies. Due to the factthat the wave length of sound in water in the lower audible frequencyregions is quite large, it becomes necessary to provide vibratingstructures whose radiating area is several square feet or greater ifsignificantly large amounts of acoustic power is to be generated. Forexample, at 300 cycles per second, a circular piston of about two feetin diameter will vibrate with a total excursion of approximately 0.011inch displacement in order to radiate one kilowatt of acoustic power. Ifthe piston diameter is reduced to about one foot the required totalpiston displacement becomes 0.044 inch. These figures are based on theassumption that the vibrating structure is a true piston and issurrounded by an infinite rigid baffle. In the absence of a battle, therequired displacement becomes even greater. It is also appreciated bythose skilled in the art that a vibrating flat plate having a diameterof a few feet would have to be reasonably thick and heavy in order forit to behave as a rigid piston during its vibratory operation. This isparticularly true because of the additional fact that the vibratingplate would be carrying along a relatively heavy water load which for atwo foot diameter structure would be approximately 150 pounds.

A practical design of an electroacoustic transducer to operatesatisfactorily under the conditions described relates to anelectromagnetically driven structure in which the vibrating pistonsurface is driven through an air gap having sufficient clearance topermit the required amplitude of vibration. In a conventionalelectromagnetic transducer wherein a rigid housing contains anelectromagnetic assembly having a vibratory piston affixed at one end,the transducer would only be capable of operating in relatively shallowwater, because the tremendous forces which are exerted on the surface ofthe piston in deeper water would soon close the air gap and render thetransducer substantially inoperative.

Prior art references for preventing air gap closure in conventionalelectromagnetic transducers have utilized pressure equalization in whichthe air pressure inside the transducer enclosure is adjusted tocorrespond to the external water pressure acting on the surface of thediaphragm. This procedure, in addition to being complicated, introducessubstantial limitations wherein the air pressure increases in the airgap and the viscosity also increases which in turn interferes with thesatisfactory operation of the transducer.

The object of this invention is to improve the design of an under-watertransducer in order to provide a vibratory structure which is capable ofellicient generation of acoustic power in relatively deep liquids, suchas water.

Another object of this invention is to improve the design of atransducer for operating under water such that the performancecharacteristics remain virtually unchanged as the depth of submergenceis varied.

A still further object of this invention is to provide a simplerelatively low-cost electromagnetic transducer design wherein truepiston operation is achieved for the vibrating portion of the transducerwithout resorting to the use of thick massive plates.

An additional object of this invention is to improve the design of aninertia-driven electromagnetic transducer by eliminating the need forelectrical conductors between the vibrating external surface of thetransducer and the internal flexibly-suspended magnetic assembly.

Another object of this invention is to design a transducer whichradiates acoustic power directly from its freely-vibrating surfaces withthe analogous efficiency and effectiveness as if the transducer wereactually mounted near the center of a very large rigid wall.

Another object of this invention is to improve the linearity of thevibratory displacement of the radiating surface of the transducer.

The novel features which are characteristic of this invention are setforth with particularity in the appended claims. The invention itself,however, both as to its organization and method of omration as well asadditional objects and advantages thereof will best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIGURE 1 is a vertical cross-sectional view of an electromagnetictransducer embodying certain novel features of the invention;

FIGURE 2 is a partial vertical sectional view showing a combination of apair of electromagnetic transducers mounted within a cylindrical bafflefor increasing the efiiciency of radiation of acoustic energy;

FIGURE 3 is a crosssectional view showing a modification of thetransducer structure of FIGURE 1 in order to achieve an efficientradiation of under-water sound energy without the need for an additionalbafiie struc ture;

FIGURE 4 is a plane view taken along line 44 in FIGURE 1, illustratingthe armature section of the transducer; and

FIGURE 5 is a perspective view partially in section of the armaturesection, as shown in FIGURE 4, illustrating the general arrangement ofthe drive coils and the extended lead terminals.

Referring more specifically to FIGURE 1, the numeral 1i) represents amassive non-magnetic base member having a general cylindrical outerperiphery and provided with a fiat surface to which is bonded by asuitable bonding material, such as an epoxy resin cement, a permanentmagnet assembly comprising of a plurality of stacks of magneticlaminations 12 interspersed with permanent magnets 14. The permanentmagnets 14 are preferably relatively thin rectangular plates of thesintered oxide type which may be efiiciently polarized through the thindimension, as illustrated by the markings N for north and S for south,representing the polarity arrangement of the various magnets. Theassembly of the lamina tion stacks 12 with the magnets 14 is achieved bycementing the various sections together into one composite structure;and then cementing one surface of the composite structure to the basemember 10 using a suitable bonding compound, such as an epoxy cement.

A stack of slotted laminations 16 is bonded to one side of a platemember 18, as shown in FIGURE 1. A corresponding number of electricalcoils 20 are rigidly bonded by cement within the slots of a laminationstack or armature 16 by a suitable bonding compound after which springmembers 22 are mounted between the surfaces of the plate member 18 andthe base member 10 by bolts 19 to establish a fixed parallel air gap 24between the ends of lamination stacks 12 and the mating 3 ends of thelamination stacks 16. The stiffness of the spring members 22 is chosento provide the desired operating frequency of the assembled transducerand the stiffness range is extremely critical in order to obtain thedesired result.

After the assembly of the composite magnetic structure with the springmembers 22 and plate member 18 and base member 10, the outer fiatsurface of plate member 18 is bonded by a suitable compound, such as anepoxy cement to the inner plane surface of the outer housing 26.

The windings of coils 20 are electrically connected in series with eachother, and the end leads of the connected group pass through an opening(not shown) provided in the plate member 18 and housing 26 and aresoldered to the terminals 28 which are mounted through an insulatingterminal bushing 30, as illustrated in FIG- URE 1. The flexible leads 31are connected to the corresponding conductors within the electricalcable 32. The cable 32 is molded to a metallic flange 34 which isattached by means of the bolts 36 to a mating member 38, which isrecessed and sealed into the housing structure 26, as shown. A highstrength bonding compound, such as an epoxy cement, may be used as asealant between the mating member 36 and the housing 26. The matingsurfaces of the flange 34 and the mating member 38 may be sealed with agasket or an O-ring. The inner terminal assembly or insulating bushing30 is mounted within a counterbored portion of housing 26. Thisconstruction prevents the entry of sea water into the working portion ofthe transducer assembly which would cause damage to the cable. The coilleads 20 which are connected to the terminals 28 pass through an openinin the housing 26 then potted and sealed with a rigid cast bondingcompound, such as an epoxy resin cement, to consolidate the windings andprevent their vibration when the transducer is operating. To completethe transducer, a complementary housing 27 is joined to the housingstructure 26 and bonded and cemented at the contacting surface 40 toprovide a composite convex hollow outer housing.

The basic electromagnetic structure described up to this point issimilar to the transducer structure described and shown in more completedetail in the copending US. patent application Serial No. 164,010, filedJanuary 3, 1962, by Frank Massa, entitled, Electroacoustic Transducer.The operation of the transducer, as described in the aforesaidapplication, takes place when the passage of alternating current throughthe coils 20 causes alternating magnetic forces at the air gap 26 whichsets up rela tive vibration between the suspended internal portion ofthe transducer and the outer shell-like housing. The generally convexhollow outer housing provides a rigid vibrating surface which can bemuch lighter in structure than the fiat vibrating surfaces and furtherwhich can resist higher pressures as are encountered in deep water. Thisinvention is not concerned with the permanent magnet electromagneticdrive system thus far described. The invention is concerned primarilywith improvements in this design, as will be outlined hereinbelow.

By attaching the drive coils 20 to the outer vibrating portion of thetransducer assembly, it is possible to eliminate the need for flexiblelead terminals for operating the transducer such as are required in thedesign shown in US. patent application Serial No. 164,010, filed January3, 1962, in which the current coils are shown attached to the innervibrating portion of the assembly. The change in construction shown inthe present invention eliminates the risk of failure of the vibratingleads during operation of the transducer.

Another improvement shown in the present design includes the use of aring member 42 which is fastened by suitable screws 44 to a recessedmachined annular surface provided in the base member 1%). The purpose ofthe ring 42 is to dynamically balance the vibrating structure which isachieved by operating the assembly before attaching housing portion 26,by measuring the relative amplitudes of vibration at various positionsaround the periphery of the ring 42. These measurements may be made byattaching several accelerometers to the face of the ring member 42 andnoting the relative displacements as measured by the severalinstruments.

The attachment of the accelerometers and the measure ment of theamplitudes of vibration is not illustrated in the drawing since thisprocedure is routine standard practice and does not form any part ofthis invention.

After reading the relative displacements at various points about theperiphery of the vibration structure, an adjustment is made by removingmaterial from the ring 42 near the region where the displacementmeasurement is higher than average. Other methods of adding orsubtracting material may be employed by plating or mechanically affixingweights. When the proper adjustment of the weight distribution of thering 42 is reached, the amplitudes of vibration at various positionsabout the periphery of the vibrating structure will be very nearly equaland the system will be dynamically balanced. The transducer structurewill now operate with vibratory mo tion which is relatively free oftwisting modes of 'viorlav tion thereby establishing a true lineardisplacement and a corresponding true simple harmonic motion of thetransducer body.

Another improvement in the design of the transducer results from the useof the disc member 46 which is bolted near its periphery to the insidering surface of the complementary housing 27 by means of screws 48, asshown. After the disc member 46 is attached to the complementary housingstructure 27, the complementary housing structure 27 is bonded to thehousing structure 26 at the peripheral region 4%, as previouslydescribed. Through a lower opening 50, a bolt 52 is fastened to the basemember 1 1 and a suitable spacer 54 is placed between the disc member 46and the base member 14) so that when the bolt 52 is tightened, theposition of the base member 19 is radially secured with respect to theassembled outer housing structure of the transducer.

The disc member 46 is rigid in a plane at right angles to the normalaxis of the transducer and is flexible along the axis of vibration ofthe transducer. In this way the disc member 46 serves to increase theruggedness of the transducer assembly so that it may withstand severeshock without damage as might otherwise occur if the massive base member10 were unsupported at its free over-hanging end. After the bolt 52 issecured, the opening 59 is sealed by a rigid plug 56 which is bonded tothe complementary housing 27 by an epoxy resin cement or some othersuitable cement.

An outer protective covering 58 of rubber or other suitable pliablematerial is preferably molded to the outer surface of the transducer toprevent corrosion of the exposed surface. The pliable covering 58 willserve to provide shear compliance to permit unrestricted oscillation ofthe transducer when it is placed in contact with a fixed surface such asthe wall of a baffle structure.

The operation of the completed transducer is as follows: the permanentmagnets 14 establish a flux density in the air gap 24 as is well knownby one skilled in the art. When an alternating current passes throughthe coils 20, the air gap flux is modulated accordingly; and. if thefrequency of the current corresponds with the mechanical resonantfrequency of the transducer, relatively large displacements will occurbetween the massive inertial base member 10 and the unitary outerhousing structure 26 and 27. The general spherical shape of thetransducer design will permit satisfactory operation in relatively deepwater because the water pressure will be resisted by the housing shape.The housing is preferably fabricated from a relatively light material,such as aluminum, magnesium or beryllium alloy compositions and the basemember 10 is preferably fabricated from relatively heavy material, suchas bronze or'non-magnetic stainless steel,

By this choice of materials it is possible to achieve relatively largeexcursions of the outer radiating surface of the transducer while theinertial base member vibrates at a lower amplitude.

If the transducer illustrated in FIGURE 1 is submerged in water, theentire housing structure will oscillate as an unitary structure. Unlessthe transducer is placed in a battle to prevent circulation from thefront to the rear portion of the oscillating structure, the acousticcoupling to the water will be relatively poor; and a relatively smallamount of acoustic power will be generated for large displacements ofthe transducer.

FIGURE 2 illustrates an arrangement of a bafiie structure which willpermit a relatively high efficient acoustic radiation from anunder-water transducer illustrated in FIGURE 1. A cylindrical pipe 661which has a clearance diameter to receive the transducers 62 is fittedwith a number of mounting brackets 64 which are secured to the wall ofthe pipe 69 by suitable bolts 66 or any other equivalent fasteningmeans. To the exposed faces of the brackets 64 is provided a resilientpad 6? which makes contact with the outer surface of transducers 62 whenthey are assembled in position. Before assembling the transducers 62, agas-filled bladder 70 or pressure release system is mounted inside thecylinder pipe (it) by means of brackets '72, as illustrated. The gas maybe air, nitrogen or any other gaseous medium. The rubber bladder 711 maybe made from two rubber sheets bonded between two metallic rings 74which, in turn, are fastened to the brackets 76 by means of the screws78. A suitable inlet valve 80 is provided in the wall of the rubberbladder 70 through which air may be pumped into the bladder to expand itto the desired volume. A protective sealing cap 82 is provided to sealthe valve stem and protect it from under water corrosion. A pair of thenovel transducers 62 are placed one in each open end of the cylinder 60,and are held in place by attaching the additional bracket members 84which are secured to the cylinder wall 60 by means of the bolts 86.Rubber pads 88 are provided on the faces of the brackets 84 to containthe transducers between resilient movement. The two transducers areelectrically connected so that they vibrate in phase; that is, bothtransducers move axially outward and inward together as they vibrate atthe ends of the cylinder.

During the operation of the batfied structure shown in FIGURE 2 acousticradiation takes place only from the outer exposed area of eachtransducer 62. The gas-filled bladder 7t) provides a suitable pressurerelease to permit free vibration of the two transducers insofar as theinner unexposed surfaces are concerned. The cylindrical bathe preventsany interference from the vibrations of the transducer surfaces facingtoward the center of the battle. The arrangement shown in FIGURE 2effectively couples the radiating exposed transducer surfaces to thewater as if an infinite wall were present in a plane bisecting the axisof the cylinder 60.

A mounting frame 90 may be welded to one end of the cylinder 60, asillustrated, and a mounting flange 92 may be provided for suspending thetransducer from a supporting cable, if desired. The transducerelectrical cable 94 is shown passing through a protective sleeve 96which is welded to the outer surface of the cylinder 60. The secondtransducer cable 98 is secured together with the free end portion oftransducer cable 94 by means of a clamp or a tape 100. Bracket members102 may be welded around the periphery of the tubular cylinder 60 to beused as pedestal mountings if the transducer assembly is to be fastenedto a flat surface such as the deck of a submarine.

FIGURE 3 shows a preferred modification of the transducer structure inwhich eflicient under-water radiation may take place without thenecessity of a special bafile. FIGURE 3 shows two permanent magnetassemblies each driving one-half of the transducer outer surfaces. Theinertial massive base member 110 to which is attached the correspondingpermanent magnet and lamination assembly 112 which serves the samefunctions as was described in connection with the structure shown inFIG- URE 1. The armature members 114 with the drive coils 116 aremounted to upper plate members 118 by means of the springs 120 tocomplete an electromagnetic assembly similar to the electromagneticassembly described in FIGURE 1. The upper plates 118 which are attachedto the housing members 122 in the same way that plate 18 was attached tohousing 26 in FIGURE 1, and the cables 124 are provided with the samefittings and the same terminal arrangements as was described for thestructure in FIGURE 1. Basically, as thus far described, FIGURE 3comprises a pair of transducer assemblies identical to the structureshown in FIGURE 1, with the exception that one portionrof the housingenclosure 26, shown in FIGURE 1, is deleted in FIGURE 3. To complete theassembly of the transducer in FIGURE 3, a bellows type spring member 126is bonded or welded to connect and to seal the two housing portions 122.

During the operation of the transducer of FiGURE 3, each housing member122 is driven in phase at the resonant frequency determined by thestiffness of the spring members 120. The bellows type spring 126 hassufficient radial stiffness to resist the water pressure at the depth ofoperation, and to permit the opposite ends of the assembled ransducer tomove together in deep water. The amount of static deflection isdetermined by the stiffness of the bellows spring 126. For maximumefficiency, it is preferable to adjust the stiffness of the springmember 126 so that it is at resonance with the radiating mass of thetransducer ends at their resonant frequency of operation. In this mannerthe internal mass members Will remain virtually stationary and themaximum displacement of the radiating surfaces 122 may be achieved for agiven air gap displacement in the transducer. This preferred stiffnessof the spring member 126 must also take into account the added mass ofthe water load which is carried into oscillation by the housing members122.

The composite transducer structure will also operate fairlysatisfactorily if the stiffness of the spring member 126 is keptrelatively low and pliable so that it will not impede the motion of theradiating faces 122 at their normal operating frequency. Under thiscondition, however, motion of the internal mass member 110 will berelatively similar to the motion displayed by the base member 10 duringthe operation of the transducer described in FIG- URE 1.

By virtue of the design in FIGURE 3, it is evident that both ends of thevibrating structure will vibrate in phase and therefore initiate soundenergy in the water without circulatory motion from one surface to theother as occurs from the oscillating structure in FIGURE 1. In otherwords, the configuration of FIGURE 3 provides within itself theequivalent of an infinite battle in a plane at right angles to the axisof vibration of the composite structure. The efficiency of radiation ofthe composite structure of FIGURE 3 will be equivalent to the efiiciencyof radiation of the bafiled transducer combination illustrated in FIGURE2.

Although a few specific examples have been chosen to illustrate thebasic principles of the invention, it will be obvious to those skilledin the art that numerous departures may be made from the details shown;and therefore it is contemplated that the invention shall not be limitedexcept insofar as is made necessary by the prior art and by the spiritof the appended claims.

What is claimed and desired to be protected by Letters Patent of theUnited States is:

1. The improvement of an alternating electroacoustic transducercomprising a sealed housing structure, including two vibratile endportions separated by a compliant member, a massive first magnetic meansadapted to be spaced from said first end portion for translatoryvibration relative thereto, a massive second magnetic means adapted tobe spaced from said second end portion for translatory vibrationrelative thereto, a third magnetic means rigidly secured to said firstend portion and positioned in operable relation to said first magneticmeans, a fourth magnetic means secured to said second end portion andpositioned in operable relation to said second magnetic means,alternating current coil generating means attached to each of said thirdand said fourth magnetic means and operatively associated with each ofsaid magnetic means, terminal means for supplying alternating electricalcurrent to said coil means, frequency determining spring elementsattached between said first and third magnetic means and frequencydetermining spring elements attached between said second and said fourthmagnetic means to hold said attached pairs of magnetic means in operablerelationship to each other whereby translatory vibration of said firstand said second end portion of said sealed housing structure will be ofthe same phase whenever alternating current is applied to the currentcoils.

2. The invention set forth in claim 1 characterized in that thecompliance of the compliant member between said vibratile end portionsis of a magnitude which is at resonance with the effective vibratorymass associated with each vibratile end portion within the frequencyband of operation of said transducer.

3. The invention set forth in claim 1 further characterized in that saidalternating current coil means are rigidly attached to said third andsaid fourth magnetic means.

4. In combination in an electroacoustic transducer adaptable forgenerating sound under water, a first transducer means comprising aninertia driven sealed housing which executes translatory vibration as aWhole upon being activated by its driving power, a second similartransducer means, a tubular member, means associated near each end ofsaid tubular member for flexibly supporting said transducer means withinsaid tubular means, and a volume of material contained within saidtubular member characterized in that its bulk modulus of elasticity isless than fifty percent of the bulk modulus of elasticity of water.

5. The invention set forth in claim 4 further characterized in that saidvolume of material comprises a water impervious flexible enclosurecontaining a gas.

6. The invention set forth in claim 4 further characterized in that eachtransducer means is operated in synchronous phase relationship such thateach end portion of the housing structure exposed to the open ends ofsaid tubular means moves simultaneously outward and inward withreference to the center of said tubular means.

7. The improvements of an inertia driven electroacoustic transducerwhich includes a sealed outer housing portion adapted for executingoscillatory vibrations, a massive inertial portion flexibly mountedinside said sealed housing portion and adapted for executing oscillatoryvibrations along the same axis of vibration of said sealed housingportion, means for dynamically balancing said vibratile inner massportion whereby all points on the surface of said vibratile inertiaportion will execute linear vibrations parallel to the axis of vibrationof said outer sealed housing structure.

8. The invention set forth in claim 7 further characterized in that saiddynamic balancing means includes weight members which may be attached atvarious regions near the periphery of said inertial mass.

References Cited by the Examiner UNITED STATES PATENTS 2,962,695 11/1960Harris 340-10 3,048,815 8/1962 Thurston et al. 3408 3,219,969 11/1965Snavely 340 12 X 3,274,538 9/ 1966 Snavely 3408 RODNEY D. BENNETT,Primary Examiner.

I. P. MORRIS, Assistant Examiner.

1. THE IMPROVEMENT OF AN ALTERNATING ELECTROACOUSTIC TRANSDUCERCOMPRISING A SEALED HOUSING STRUCTURE, INCLUDING TWO VIBRATILE ENDPORTIONS SEPARATED BY A COMPLAINT MEMBER, A MASSIVE FIRST MAGNETIC MEANSADAPTED TO BE SPACED FROM SAID FIRST END PORTION FOR TRANSLATORYVIBRATION RELATIVE THERETO, A MASSIVE SECOND MAGNETIC MEANS ADAPTED TOBE SPACED FROM SAID SECOND END PORTION FOR TRANSLATORY VIBRATIONRELATIVE THERETO, A THIRD MAGNETIC MEANS RIGIDLY SECURED TO SAID FIRSTEND PORTION AND POSITIONED IN OPERABLE RELATION TO SAID FIRST MAGNETICMEANS, A FOURTH MAGNETIC MEANS SECURED TO SAID SECOND END PORTION ANDPOSITIONED IN OPERABLE RELATION TO SAID SECOND MAGNETIC MEANS,ALTERNATING CURRENT COIL GENERATING MEANS ATTACHED TO EACH OF SAID THIRDAND SAID FOURTH MAGNETIC MEANS AND OPERATIVELY ASSOCIATED WITH EACH OFSAID MAGNETIC MEANS, TERMINAL MEANS FOR SUPPLYING ALTERNATING ELECTRICALCURRENT TO SAID COIL MEANS, FREQUENCY DETERMINING SPRING ELEMENTSATTACHED BETWEEN SAID FIRST AND THIRD MAGNETIC MEANS AND FREQUENCYDETERMINING SPRING ELEMENTS ATTACHED BETWEEN SAID SECOND AND SAID FOURTHMAGNETIC MEANS TO HOLD SAID ATTACHED PAIRS OF MAGNETIC MEANS IN OPERABLERELATIONSHIP TO EACH OTHER WHEREBY TRANSLATORY VIBRATION OF SAID FIRSTAND SAID SECOND END PORTION OF SAID SEALED HOUSING STRUCTURE WILL BE OFTHE SAME PHASE WHENEVER ALTERNATING CURRENT IS APPLIED TO THE CURRENTCOILS.